This invention relates to improvements in known processes for preparing ferromagnetic chromium dioxide that afford reduced process time requirements in commercial-scale operations.
Ferromagnetic materials have come into extensive use over the past few decades for magnetic recording tapes, discs and drums and the like as well as for memory devices, microwave circuitry, and magnetic cores. During this same period, many processes have been developed for the production of ferromagnetic chromium dioxide. In one such process, described in U.S. Pat. No. 2,956,955 issued Oct. 18, 1960 to Paul Arthur, Jr., a chromium (VI) oxide, CrO.sub.3, is hydrothermally reduced to chromium (IV) oxide, CrO.sub.2. In another process, described in U.S. Pat. No. 3,278,263 issued Oct. 11, 1966 to Norman L. Cox, compounds of chromium (III) combined with oxygen are oxidized to chromium (IV) oxide, CrO.sub.2, by such oxidizing agents as O.sub.2, H.sub.2 O.sub.2 and CrO.sub.3. For the Cox process, chromium (III) oxide, or a hydrated form thereof, is preferred because such materials are readily available and are easily converted to high-quality ferromagnetic chromium dioxide. However, other compounds of chromium (III) combined with oxygen, e.g., chromium (III) hydroxide, or their hydrates, can be employed as starting materials if desired.
The conversion of these starting materials to ferromagnetic chromium dioxide is customarily carried out at temperatures above 250.degree. C., since conversion to ferromagnetic chromium dioxide at lower temperatures is very slow and usually incomplete. Although temperatures as high as 500.degree. C. and above can be employed, such temperatures require the application of excessively high pressures and usually are to be avoided. Products of excellent magnetic properties are obtained when a maximum reaction temperature in the range of 300.degree.-450.degree. C. is employed.
The pressures employed in the processes of these prior art teachings usually range from about 50 atmospheres to 3000 atmospheres or more. Pressures of 50 to 800 atmospheres are preferred, since these pressures can be obtained more economically than higher pressures. The resulting chromium dioxide is then separated, dried and pulverized for its ultimate application.
For the preparation of small amounts of chromium dioxide on a laboratory scale, the reactions involved in these processes can be carried to completion in times as short as 10 minutes or less. On the other hand, when these processes are carried out on a commercial scale, it is customary to use pressure vessels or autoclaves that are circular in cross-section and usually cylindrical in overall form for the reason that this configuration is most suitable to withstand the mechanical stresses encountered at the operating pressures involved. To maximize productivity and operating economy of this inherently expensive equipment, it has been conventional to maximize the "charge," i.e., to fill the pressure vessel to the greatest possible extent with reactant materials in amounts that may reach several hundred pounds. Ordinarily, this is accomplished by loading the reactant materials in a reaction vessel having the form of a right cylinder. A plurality of such reaction vessels are then placed in stacked array in the pressure vessel and heated so that the desired hydrothermal reaction can take place. Because of the large mass of reactant material and the relatively inefficient heat transfer implicit in the loading arrangement, such a commercial-scale process may typically require a prolonged heating time of the order of 20 hours or more.
Once the reaction is complete, the pressure vessel is typically cooled and vented to allow excess steam and oxygen to escape. Following this, the reaction vessels are removed from the pressure vessel and dried, usually in an oven. This cooling, venting and drying typically takes on the order of 50 hours or more, in part because of the poor mass transfer of the water out of the reaction product and in part because of the poor heat transfer by which the product is permitted to cool and dry.
It is, therefore, an object of this invention to provide an improved process for the preparation of ferromagnetic chromium dioxide in which the processing times are reduced.